1. Field of the Invention
This invention relates to the field of communications transmissions and to a method for extracting the DC component of an analog input signal.
2. Background Art
Communication of data information over the voice network between a sending and receiving station is typically accomplished with a modem (modulator/demodulator). A modem transforms digital information into a form suitable for transmission by modulating it according to a well known modulating scheme. This modulated data is transmitted to a receiving modem, where it is demodulated to produce the original information.
One popular modulation scheme is known as quadrature amplitude modulation (QAM) in which the phase and amplitude of a modulated sine wave carrier signal are utilized to convey information. The QAM signal consists of quadbits which are mapped into a 16 point constellation. Phase and amplitude errors may be introduced into the signal due to analog circuitry associated with the modulation and demodulation of the signal. Typically, an automatic gain control (AGC) circuit is utilized to compensate for variations in amplitude. In addition, a phase locked loop is utilized to correct phase variations. In addition to phase and amplitude errors, there may be introduced to an input signal, a component which creates a DC offset in the QAM constellation. This DC offset is not correctable by prior art phase and amplitude correction methods and therefore requires a separate correction scheme. If not corrected, the DC offset introduces error into the decoding of the information signals.
Therefore, it is an object of the present invention to provide a circuit for correcting DC offset in an analog input signal.
It is another object of the present invention to provide a circuit for correcting DC offset which is substantially decoupled from phase correction and amplitude correction.
It is another object of the present invention to provide a circuit for correction of DC offset and having a resolution suitable for information transmission applications.